It is a well-known procedure for hollow bodies to be produced from thermoplastic material by blow molding, wherein a preform which is for example of a tubular configuration is formed and expanded within a blow molding mold by an increased internal pressure therein, until it bears against the wall of the blow molding mold. Some situations however involve the need for an additional body to be disposed in the interior of the hollow body which is produced by that blow molding procedure, for example typical situations of use in that respect being motor vehicle tanks which, if they are made from plastic material, are predominantly produced by an extrusion blow molding procedure. The additional body may be a surge cup or pot which is disposed in the lower region of the tank and which is intended in particular to ensure that sufficient fuel is always available in the region of the intake opening leading to the fuel pump, irrespective of forces acting on the fuel in the tank, for example when the vehicle in which the tank is fitted is negotiating a bend, thus being subjected to centrifugal forces.
In the majority of procedures for disposing an additional body in the interior of a hollow body produced by blow molding, upon extrusion of the preform, the latter is passed over the additional body which is supported by a carrier element, whereupon, upon closure of the blow molding mold which is generally in two parts around the preform, the additional body is enclosed within the preform which, at that time, has generally already been subjected to preliminary expansion. Such preforms are predominantly of a tubular configuration, so that reference may be specifically made hereinafter to tubular preforms, although it will be appreciated that the teaching of the present invention can also be readily applied to preforms of different configurations.
After the preform has been expanded into the configuration of the hollow body to be produced therefrom, the additional body is joined to the internal surface of the wall of the hollow body. That is generally effected by the additional body being welded or heat-sealed to the wall of the hollow body. The additional body at least partly comprises a material which is weldable to the material forming the hollow body. Such a welded joint can be made for example when the hollow body and the additional body at the join locations comprise polyolefins, for example polyethylene. For that purpose, at the moment of producing the join between the additional body and the wall of the hollow body, those components must still be hot and plastic, that is to say in a condition which permits a welded join to be produced therebetween.
As indicated above, a typical additional body which is to be fitted into a hollow body is a surge cup or pot provided in the interior of a motor vehicle fuel tank and intended to ensure that fuel is retained in the region of the intake opening leading to the fuel pump, thereby to ensure that fuel is always available to the pump. In such a situation, the intake opening for the fuel line leading to the fuel pump is generally disposed in the surge pot which in substance comprises a bottom portion and wall regions which extend therefrom generally perpendicularly to the bottom portion but which are provided with openings for the fuel to pass therethrough, into the surge pot. The surge pot is generally open at its side remote from the bottom portion. Surge pots of that kind are generally known and will therefore not be described in greater detail at this point.
As the walls of fuel tanks which comprise a polyolefin such as more particularly polyethylene are permeable at any event in relation to those hydrocarbons which are contained in liquid fuels, the usual practice is for the wall of the fuel tanks to be exposed to a reactive gas, prior to use of the tank. The reactive gas used in that situation is generally a fluorine-bearing gas mixture which is blown into the hollow body and which results in reaction of the internal surface layer of the wall consisting of polyethylene or the like, with the result that that surface layer and therewith the wall generally becomes almost impermeable in relation to hydrocarbons, or at any event is of such a low degree of permeability that the hydrocarbons which still pass through the wall of the tank are insignificant in quantitative terms.
Hitherto the procedure for the production of a hollow body with an additional body mounted therein is generally such that, after the preform expansion operation, the additional body is firstly joined to the wall of the hollow body, whereupon the treatment with fluorine or another treatment for reducing the level of permeability in relation to hydrocarbons is then carried out. That means that the region of the inside surface of the wall at which the additional body is mounted is shielded from the reaction medium by the additional body, so that that region which is shielded during the treatment with the reaction medium experiences no or only slight reaction of the surface layer of the wall of the hollow body and the degree of permeability of that region therefore remains at least substantially unchanged. That disadvantageous effect could possibly be at least reduced in some areas, that is to say where the additional body does not bear against the wall of the hollow body, by arranging for the reaction medium to act for a longer period of time. That however would result in the treatment time being increased, giving rise to a correspondingly lower level of productivity.
Such a procedure suffers from disadvantages in particular when using a conventional surge pot or cup in a motor vehicle fuel tank, as the cup or pot is generally provided with a bottom portion of large area, which boars directly against the inside surface of the wall of the hollow body and thus covers a correspondingly large region thereof and causes that region to remain untreated by the reaction medium. Admittedly, that untreated region of the wall of the hollow body does not necessarily result in its being permeable in relation to hydrocarbons, which would make it impossible for the hollow body to be used as a tank, especially as the presence of the bottom portion of the surge cup or pot in that region means that the total wall configuration at that location, being constituted by the wall of the hollow body plus the bottom portion of the surge cup or pot, is thicker than in the remainder of the tank. However, the absence of surface treatment in that region means that in use of the tank, in the course of time, hydrocarbons from the fuel contained in the tank can penetrate into that region of the wall of the tank. The result of that inter alia is that the regions of the wall of the tank through which the hydrocarbons pass may experience a certain degree of deformation, for example due to swelling thereof. The degree of such deformation does not need to be very great but at any event it gives rise to the occurrence of stresses in the wall, which are undesirable and which can also result in deformation of wall regions of the tank; such deformation can also produce an effect right up to the additional body, causing a change in the position thereof in the tank, so that operational components in the additional body may under some circumstances be affected in a manner such as to impair proper functioning thereof. It will be appreciated that the above-mentioned hydrocarbons may also penetrate into the bottom and wall regions of a surge pot or cup which comprises for example polyethylene, and may also cause swelling at those locations. That however is of no significance in relation to the surge cup or pot as it is not subjected to high mechanical loadings and in addition is arranged within the tank. Admittedly the reaction medium for treating the internal surface of the tank also acts on the surge pot or cup--or any other additional body disposed within a hollow body--, although without producing the effects on the surfaces of the additional body, which occur at the internal surface of the hollow body. That is in substance to be attributed to the consideration that the additional body is at a substantially lower temperature than the wall of the hollow body, which is still hot and plastic. The temperature of the surfaces to be treated in the above-described manner represents an essential requirement in regard to achieving the desired effect.